National Institute Of Materials Physics - Romania

Publications

490 articles found

481. Reactivity and magnetism of Fe/InAs(100) interfaces

Authors: Teodorescu, CM; Chevrier, F; Brochier, R; Richter, C; Ilakovac, V; Heckmann, O; De Padova, P; Hricovini, K

Published: AUG 2002, EUROPEAN PHYSICAL JOURNAL B, 28, 313, DOI: 10.1140/epjb/e2002-00234-6

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

482. Reducibility of ruthenium in relation with zeolite structure

Authors: Parvulescu, VI; Coman, S; Palade, P; Macovei, D; Teodorescu, CM; Filoti, G; Molina, R; Poncelet, G; Wagner, FE

Published: MAR 1999, APPLIED SURFACE SCIENCE, 141, 176, DOI: 10.1016/S0169-4332(98)00619-9

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

483. EXAFS CHARACTERIZATION OF DY AND PD-DY ON ALUMINA CATALYSTS

Authors: MACOVEI, D; PARVULESCU, V; TEODORESCU, CM

Published: APR 1994, REACTION KINETICS AND CATALYSIS LETTERS, 52, 86, DOI: 10.1007/BF02129853

Two Pd-Dy/gamma-Al2O3 catalysts, with 1 wt.% Dy and various Pd loadings (0.1 and 0.5 wt.%) have been investigated by EXAFS at the Dy L(III)-edge, and compared with 1 % Dy/gamma-Al2O3. The Dy-carrier interaction was found to change with the Pd dispersity.

484. MULTIPHOTON IONIZATION OF RUBIDIUM

Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; CHELLEHMALZADEH, MA; ANDERSON, JA; POPESCU, D; POPESCU, I

Published: 1976, PHYSICAL REVIEW A, 14, 1671, DOI: 10.1103/PhysRevA.14.1662

485. FINE-STRUCTURE MEASUREMENTS OF 2-PHOTON TRANSITIONS IN ATOMIC CESIUM WITH A TUNABLE DYE LASER

Authors: CURRY, SM; COLLINS, CB; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1976, OPTICS COMMUNICATIONS, 16, 255, DOI: 10.1016/0030-4018(76)90229-7

486. MULTIPHOTON EXCITATION OF ATOMIC RUBIDIUM WITH A TUNABLE DYE LASER

Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1975, BULLETIN OF THE AMERICAN PHYSICAL SOCIETY, 20, 246, DOI:

487. DOUBLET LINE-STRENGTH RATIOS FOR PRINCIPAL SERIES OF CESIUM

Authors: POPESCU, D; POPESCU, I; MAURER, J; COLLINS, CB; JOHNSON, BW

Published: 1975, PHYSICAL REVIEW A, 12, 1431, DOI: 10.1103/PhysRevA.12.1425

488. Sonogashira Synthesis of New Porous Aromatic Framework- Entrapped Palladium Nanoparticles as Heterogeneous Catalysts for Suzuki-Miyaura Cross-Coupling

Authors: Cata, L; Terenti, N; Cociug, C; Hadade, ND; Grosu, I; Bucur, C; Cojocaru, B; Parvulescu, VI; Mazur, M; Cejka, J

Published: , ACS APPLIED MATERIALS & INTERFACES, DOI:

Palladium nanoparticles entrapped in porous aromatic frameworks (PAFs) or covalent organic frameworks may promote heterogeneous catalytic reactions. However, preparing such materials as active nanocatalysts usually requires additional steps for palladium entrapment and reduction. This paper reports as a new approach, a simple procedure leading to the self-entrapment of Pd nanoparticles within the PAF structure. Thus, the selected Sonogashira synthesis affords PAF-entrapped Pd nanoparticles that can catalyze the C-C Suzuki-Miyaura cross-coupling reactions. Following this new concept, PAFs were synthesized via Sonogashira cross-coupling of the tetraiodurated derivative of tetraphenyladamantane or spiro-9,9 '-bifluorene with 1,6-diethynylpyrene, then characterized them using powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, high-resolution scanning transmission electron microscopy, and textural properties (i.e., adsorption-desorption isotherms). The PAF-entrapped Pd nanocatalysts showed high catalytic activity in Suzuki-Miyaura coupling reactions (demonstrated by preserving the turnover frequency values) and stability (demonstrated by palladium leaching and recycling experiments). This new approach presents a new class of PAFs with unique structural, topological, and compositional complexities as entrapped metal nanocatalysts or for other diverse applications.

489. Nanocrystalline rhenium-doped TiO2: an efficient catalyst in the one-pot conversion of carbohydrates into levulinic acid. The synergistic effect between Bronsted and Lewis acid sites

Authors: Avramescu, S; Ene, CD; Ciobanu, M; Schnee, J; Devred, F; Bucur, C; Vasile, E; Colaciello, L; Richards, R; Gaigneaux, EM; Verziu, MN

Published: , CATALYSIS SCIENCE & TECHNOLOGY, DOI: 10.1039/d1cy01450a

Catalytic activity of TiO2, 2%Re-TiO2 and 10%Re-TiO2 in the conversion of carbohydrates into levulinic acid under autoclave conditions was evaluated. These materials were prepared by aerogel method, for the first time to the best of our knowledge, and characterized by XPS, SEM-EDX, DRIFTS, DR UV-vis, Raman, N-2 adsorption/desorption isotherms, TGA and XRD. Further, the surface acidity was probed by NH3-TPD and pyridine-FT-IR where it was observed that increasing the amount of rhenium doped into TiO2 led to an increase in the total number of acid sites (Lewis + Bronsted) but with an overall lower strength. The presence of both Bronsted and Lewis acid sites led to the hypothesis that these materials may be well suited for conversion of carbohydrates into levulinic acid. Indeed a levulinic acid yield of 57% was reached over 10%Re-TiO2 for a low mass ratio catalyst to glucose (1 : 5). Moreover, the 10%Re-TiO2 catalyst was reused in the conversion of glucose for four catalytic cycles without a significant loss of the catalytic activity.

490. Role of vanadium oxide on the lithium silicate glass structure and properties

Authors: Gaddam, A; Allu, AR; Fernandes, HR; Stan, GE; Negrila, CC; Jamale, AP; Mear, FO; Montagne, L; Ferreira, JMF

Published: , JOURNAL OF THE AMERICAN CERAMIC SOCIETY, DOI: 10.1111/jace.17671

The structural role of V in 28Li(2)O-72SiO(2) (in mol%) lithium silicate glass doped with 0.5 mol% V2O5 was assessed using Si-29 and V-51 Nuclear Magnetic Resonance (NMR), Fourier-transform infrared (FTIR), and X-ray photoelectron (XPS) spectroscopy techniques. Despite the low amount of V2O5 used, the structural information obtained or deduced from the statistical analysis of the NMR data could explain the evolution of glass properties after V2O5 addition. The XPS results indicated that all vanadium exists in 5+ oxidation state. Both the Si-29 NMR and FTIR data point toward an increase in the polymerization of the silicate network, caused by the V2O5 acting as network former, capable to form various QVn tetrahedral units (for n = 0, 1, and 2) in the glasses. These QVn units, which are similar to phosphate units, scavenge the Li+ ions and cause the silicate network to polymerize. However, in an overall balance, the entire glass network is depolymerized due to the additional nonbridging oxygens contributed by the vanadium polyhedra. The addition of vanadium causes the network to expand and increases the ionic conductivity.